A reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.
Various approaches have been taken in the past to protecting electronic equipment from potentially damaging voltage surges due to, for example, lightning strikes and power supply malfunctions.
These approaches include the use of circuits incorporating metal oxide varistors (MOVs) and gas discharge tubes to short surges to ground before they are able to damage connected equipment. Such circuits are typically bulky and may have large associated capacitances which are undesirable.
A circuit which does not make use of gas discharge tubes is described in U.S. Pat. No. 4,695,916 and reproduced herein as FIG. 1. The circuit 2 includes a bi-directional thyristor 4, or as it is sometimes called a “diac”, in series with a bi-directional zener diode 6. Alternatively a Sidac may be preferred in some applications. The SIDAC is a silicon bilateral voltage triggered switch with greater power handling capabilities than standard DIACs. Upon application of a voltage exceeding the SIDAC breakover voltage point, the SIDAC switches on through a negative resistance region to a low on-state voltage. Conduction will continue until the current is interrupted or drops below the minimum holding current of the device.
Zener diode 6 has a breakdown voltage VB slightly higher than power supply voltage Vps on signal line 10. The sum (VBO+VB) of breakover voltage VBO of diac 4 and breakdown voltage VB is set to be lower than the breakdown voltage VR of equipment 8 but is higher than that equipment's minimum operational voltage Va. In the event of a surge being applied to line 10 then the voltage across circuit 2 will rise. Upon the voltage across circuit 2 reaching (VBO+VB) the diac turns on effectively shorting zener diode 6 to rail 10. At the same time zener diode 6 becomes voltage limiting. The voltage between line 10 and ground is then held well below VR as it is the sum of the fixed breakdown voltage VB of the zener diode plus the operating voltage of the diac which is typically a few volts.
Whilst the circuit of FIG. 1 is relatively quick-acting and overcomes the problems associated with the use of gas discharge tubes, problems are encountered in the event of the surge being sustained. For example, in U.S. Pat. No. 4,695,916 it is explained that where protection from lightning surges lasting 1-msec is required then the diac may be made having a chip area as small as about 4 mm2. However, surges may last longer than 1-msec. For example, a surge due to malfunction of power supply equipment coupled to line 10 may last for several seconds. In that case circuit 2 would likely be destroyed whilst dissipating power associated with the surge in the form of heat. The destruction of circuit 2 during a surge would leave equipment 8 vulnerable to damage which is highly undesirable.
In light of the above, it is an object of the present invention to provide an electronic surge protection apparatus which is an improvement on the prior art devices described thus far.